The present invention relates generally to electronic printing systems, and more specifically to an electronic screening system for halftone color images.
As described in Japanese Provisional Specification (Tokkaisho) 61-186077, an electronic screening system comprise a color scanner that scans a color halftone original to produce digital video signals, one for each different colors, yellow, magenta, cyan and black. The color video signals are respectively stored in video memories. For screening the stored video signals, an electronic equivalent of photographic screens is overlaid on the video image of the corresponding color at an angle called "screen angle" relative to the orientation of the underlying image. Electronic screens are represented by "screen values", and different matrices of screen values are prepared for the different screen angles and stored in a screen memory as decision thresholds. To reduce the size of the screen memory, screen values are selected from adjoining repeating areas. In a read mode of the system, each video memory is accessed line-by-line and the screen memory is repeatedly accessed to compare the gradation value of the retrieved video signal with the screen value pixel-by-pixel to produce a black dot signal or a white dot signal depending on the relative magnitudes of the compared values. The dot signals are used to reconstruct images in an output memory one for each of the different colors and supplied to a printing device in which the reconstructed color images are overlaid one upon another to produce a replica of the halftone color original.
It is known in the art of photographic screen printing that the best screen angle at which Moire fringe patterns are much less visible is 0.degree. for yellow, 15.degree. for magenta, -15.degree. for cyan, and 45.degree. for black with respect to the direction of line shift. Since it is impossible to implement the screen angles precisely at 15.degree. and -15.degree., the screen angle of the prior art system is 18.5.degree. for magenta and -18.5.degree. for cyan for practical purposes. However, deviations from these ideal screen angles are a potential source of undesirable Moire fringe patterns. One approach would be to use rational tangent values to approximate the ideal screen angles with resultant large and differing matrix sizes for different colors which produce Moire fringe patterns in the printed image.